Two-color electrophoretic printing



5,1968 15. J. J. LENNON 3,409,528

TWO-COLOR ELECTROPHORETI C PRINTING Filed Aug. 2, 1965 INVENTOR DONALD J. J. LENNON ATTORNEYS United States latent O ABSTRACT OF THE DISCLOSURE An electrophoretic two-color printing system employs a source sheet containing undissolved oppositely charged color materials, and electrodes which may be polarized in either direction. The color materials may be adsorbed on an adsorbent material or undissolved in a liquid medium in which they are not soluble.

This is a continuation-in-part of my copending application Ser. No. 338,245, filed Jan. 16, 1964, now US. Patent No. 3,372,102. This invention relates to electrophoretic printing and in particular to printing in either of two colors.

Eelectrophoretic printing is described in Oster, United States Patent No. 3,145,156, and consists generally in causing selective electrophoretic transfer of charged color particles from a source sheet to a contiguous recipient sheet. In carrying out the process, a source sheet containing charged or ionizable color particles and wet with an appropriate liquid, is placed in face to face contact with a similarly moistened recipient sheet. The two sheets are then placed between electrodes and an electrical field is applied selectively to cause the electrophoretic transfer of color particles from the source sheet to the recipient sheet, the shape of the field controlling the shape of the pattern printed.

In practice, it has been found that when the moistened source sheet and recipient sheet are placed together, there is a natural tendency for the particles of color material to migrate by diffusion into the recipient sheet. Similarly, if particles of opposite charge are present in the same sheet, they may tend to agglomerate into uncharged masses.

My copending application, Ser. No. 338,245, filed Jan. 16, 1964, describes how unwanted migration of color particles can be avoided by causing the color particles to be adsorbed by an adsorbent material, which serves to hold them until they are dislodged by the electrophoretic field.

The copending application of Donald J. I. Lennon and Harvey G. Schleifstein, application Ser. No. 476,424, filed concurrently herewith describes how unwanted migration in a moistened source sheet can be prevented using a moistening liquid compounded in such a manner that the color material is not soluble sufiiciently to migrate until the electrical field is applied. The dyestuif may be brought into contact with a mixture of a solvent and a nonsolvent in proportions such that the dyestuff is not soluble and will not diffuse or migrate until an electrical field is applied. Under those conditions it appears that the energy of the field becomes a factor in placing the color material in condition for electrophoretic migration.

The electrophoretic printing processes described in the aforementioned applications have been developed to the point where they are applicable to high speed printing, for instance for computer read out use. For such purposes an additional dimension of information is provided if the color of the printed information could be selected, for instance to differentiate readily between debit and credit entries in accounting processes.

According to the present invention two color printing is provided by utilizing a source sheet containing two differently colored materials of different sign of charge, at least one, and preferably both, being essentially undissolved in the moistening liquid. The apparatus features printing electrodes energized from a voltage source'of reversible polarity.

The source sheet may be made as described in either of the two aforesaid applications, that is to say by adsorbing the dye stuffs on an adsorbent such as activated charcoal, which is then coated onto the source sheet, or by impregnating the source sheet with oppositely charged dyes, and then employing a moistening liquid which is a nonsolvent for at least one and preferably both of the dyes.

In a preferred embodiment a source sheet was made by impregnating a standard nylon typewriter ribbon .004 inch thick with a methyl alcohol solution containing 30 percent 'by weight of a mixture of:

Parts by weight 5 Iosol Black 1 Iosol Red 2 3 Iosol Yellow 3 1 Solvent black 13. anionic. 2 Solvent red 68, anionic. 3 Solvent yellow 42, anionic.

and 0.3 percent by weight of Fuchsine N. The ribbon was dried preparatory to use.

vPrior to use, the ribbon, and a recipient sheet of paper .002 inch thick were both remoistened with a mixture containing 22% by weight of methyl ethyl ketone and 78% water. The sheets were arranged in face to face contact, placed between the printing electrodes, spaced apart by 0.008 inch and energized with a two millisecond pulse of 600 volts D.C. With the electrode adjacent to the recipient sheet the anode printing was in black, and when the cathode, printing was in red. A print corresponding to the anode characters was formed, at rates corresponding to 10,000 lines per minute.

The printing apparatus is shown schematically in cross section in the accompanying drawing.

The source sheet 10 and recipient sheet 12, both moistened as described above, are placed between a base electrode plate 14, and a printing elect-rode 16 having exposed conductive characters 18. The printing electrode consists of a brass type slug 20 having raised portions 22 defining the characters 18, embedded in an epoxy resin casing 24 with the character faces exposed. Conveniently the printing electrode is formedby putting the type slug 20 in the epoxy casing 24 and then grinding away the printing face until the characters 18 are exposed.

A voltage source 26 connects to the electrodes 14 0 and 16 by leads 28, and 30, through a polarity reversing switching device 32.

The switching device 32 and source 26 may together comprise a square wave generator, and means (not shown) may also be provided for advancing the sheets 10 and 12 for multiple high speed printing.

Although this invention has been described with reference to its preferred embodiment, it is contemplated that those skilled in the art and familiar with the principles herein disclosed will readily make obvious modifications without departing from the scope of this invention. One will readily select other dyes, and other solvent systems for them and determine by routine experimentation their relative utilities in the practice of this invention. It is in this connection not contemplated that the invention is limited. to particular dyes, and particular solvent systems,

4 Cl 42510, cationic,

since others may readily be selected by those skilled in the art on the basis of this disclosure.

Having thus disclosed my invention and described in detail its preferred embodiment, I claim and desire to secure by Letters Patent:

1. A process of electrophoretic printing which comprises providing a source sheet containing an adsorbent material having at least two undissolved color materials of opposite sign of charge each of which is capable of electrophoretic transfer adsorbed on said adsorbent material placing a recipient sheet in face to face contact with said source sheet between a pair of electrodes, said sheets containing a liquid medium for electrophoretic migration, applying a voltage across said electrodes to establish an electrical field between them effective to cause electrophoretic migration of one of said color materials from said source sheet to said recipient sheet, and reversing the polarity of said electrodes to cause electrophoretic migration of another of said color materials from said source sheet to said recipient sheet when said voltage is applied.

2. A process of electrophoretic printing which comprises providing a source sheet containing at least two color materials of different color and of opposite sign of charge, each being undissolved, and each being capable of electrophoretic transfer, placing a recipient sheet in face to face contact with said source sheet between a pair of electrodes, said sheets containing a liquid medium for electrophoretic migration in which said color materials are not soluble, applying a voltage across said electrodes to establish an electrical field between them effective to cause electrophoretic migration of one of said color materials from said source sheet to said recipient sheet, and reversing the polarity of said electrodes to cause electrophoretic migration of another of said color materials from said source sheet said recipient sheet when said voltage is applied.

HOWARD S. WILLIAMS, Primary Examiner. E. ZAGAR'ELLA, Assistant Examiner. 

